All of the protein kinases that have been identified to date in the human genome share a highly conserved catalytic domain of around 300 aa. This domain folds into a bi-lobed structure in which reside ATP-binding and catalytic sites. The complexity of protein kinase regulation allows many potential mechanisms of inhibition including competition with activating ligands, modulation of positive and negative regulators, interference with protein dimerization, and allosteric or competitive inhibition at the substrate or ATP binding sites.
Axl (also known as UFO, ARK, and Tyro7; nucleotide accession numbers NM_021913 and NM_001699; protein accession numbers NP_068713 and NP_001690) is a receptor protein tyrosine kinase (RTK) that comprises a C-terminal extracellular ligand-binding domain and N-terminal cytoplasmic region containing the catalytic domain. The extracellular domain of Axl has a unique structure that juxtaposes immunoglobulin and fibronectin Type III repeats and is reminiscent of the structure of neural cell adhesion molecules. Axl and its two close relatives, Mer/Nyk and Sky (Tyro3/Rse/Dtk), collectively known as the Tyro3 family of RTKs, all bind and are stimulated to varying degrees by the same ligand, Gas6 (growth arrest specific-6), a ˜76 kDa secreted protein with significant homology to the coagulation cascade regulator, Protein S. In addition to binding to ligands, the Axl extracellular domain has been shown to undergo hemophilic interactions that mediate cell aggregation, suggesting that one important function of Axl may be to mediate cell-cell adhesion.
Axl is predominantly expressed in the vasculature in both endothelial cells (EC's) and vascular smooth muscle cells (VSMC's) and in cells of the myeloid lineage and is also detected in breast epithelial cells, chondrocytes, Sertoli cells and neurons. Several functions including protection from apoptosis induced by serum starvation, TNF-α or the viral protein E1A, as well as migration and cell differentiation have been ascribed to Axl signaling in cell culture. However, Axl−/− mice exhibit no overt developmental phenotype and the physiological function of Axl in vivo is not clearly established in the literature.
Angiogenesis (the formation of new blood vessels) is limited to functions such as wound healing and the female reproductive cycle in healthy adults. This physiological process has been co-opted by tumors, thus securing an adequate blood supply that feeds tumor growth and facilitates metastasis. Deregulated angiogenesis also a feature of many other diseases (for example, psoriasis, rheumatoid arthritis, endometriosis and blindness due to age-related macular degeneration (AMD), retinopathy of prematurity and diabetes) and often contributes to the progression or pathology of the condition.
The overexpression of Axl and/or its ligand has also been reported in a wide variety of solid tumor types including, but not limited to, breast, renal, endometrial, ovarian, thyroid, non-small cell lung carcinoma, and uveal melanoma as well as in myeloid leukemias. Furthermore, it possesses transforming activity in NIH3T3 and 32D cells. It has been demonstrated that loss of Axl expression in tumor cells blocks the growth of solid human neoplasms in an in vivo MDA-MB-231 breast carcinoma xenograft model. Taken together, these data suggest Axl signaling can independently regulate EC angiogenesis and tumor growth and thus represents a novel target class for tumor therapeutic development.
The expression of Axl and Gas6 proteins is upregulated in a variety of other disease states including endometriosis, vascular injury and kidney disease and Axl signaling is functionally implicated in the latter two indications. Axl-Gas6 signaling amplifies platelet responses and is implicated in thrombus formation. Axl may thus potentially represent a therapeutic target for a number of diverse pathological conditions including solid tumors, including, but not limited to, breast, renal, endometrial, ovarian, thyroid, non-small cell lung carcinoma and uveal melanoma; liquid tumors, including but not limited to, leukemias (particularly myeloid leukemias) and lymphomas; endometriosis, vascular disease/injury (including but not limited to restenosis, atherosclerosis and thrombosis), psoriasis; visual impairment due to macular degeneration; diabetic retinopathy and retinopathy of prematurity; kidney disease (including but not limited to glomerulonephritis, diabetic nephropathy and renal transplant rejection), rheumatoid arthritis; osteoporosis, osteoarthritis and cataracts.